1. Field of the Invention
This invention relates to an engine starter used to start an internal combustion engine, and in particular, to an engine starter equipped with a magnet switch.
2. Description of the Related Art
In a magnet switch provided in a conventional auxiliary rotation type engine starter, there are provided a main contact and an auxiliary contact in order to control the operation of the starter motor at the time of engine start in two stages, and a resistor is connected in series to a circuit formed by closing the auxiliary contact, connecting a battery and the armature of the motor to each other. On the other hand, a circuit formed by closing the main contact allows direct connection of the battery and the armature of the motor. When the start switch is turned on, an electric current is caused to flow to an attraction coil, and the auxiliary contact is closed, the electric current flowing from the battery to the motor armature by way of the resistor, so that a pinion provided on the motor shaft and a ring gear are smoothly engaged with each other, and then the main contact is connected to effect transition to rated rotation. There are provided different plungers respectively for the main contact and the auxiliary contact (See, for example, JP 7-109967 A, paragraph 0009, FIG. 1).
However, since the main contact and the auxiliary contact are driven respectively by the different plungers, the size of the starter has to be rather large, which is disadvantageous from the viewpoint of the layout of the terminals and the peripheral components. In view of this, of the main contact and the auxiliary contact, the movable one is used in common, and, at the same time, the resistor is also contained in the magnet switch to thereby achieve a reduction in size (See, for example, JP 7-174062 A, paragraph 0009, FIG. 2).
However, in a magnet switch in which a retraction coil and a holding coil are separately provided, the retraction coil and the holding coil are constructed in the same form in order to avoid the holding force of the movable element due to the induced back electromotive force after the opening of the ignition switch, with the result that the retracting speed of the movable element is increased. Thus, the coil for driving the movable element is formed by a single stage of attraction coil having high inductance, and the resistor contained in the magnet switch is connected in series to the auxiliary contact, the main contact and the auxiliary contact being opened and closed stepwise to cause an electric current to flow through the armature of the motor (See, for example, JP 2001-508855 A).
In such a magnet switch, which has a construction in which the movable one of the main contact and the auxiliary contact held by the same plunger is axially moved, the auxiliary contact is further pushed in after its turning on until the main contact is turned on, so that the push-in amount of the auxiliary contact in rated rotation is rather large. When the start switch is turned off, the plunger is returned by a distance corresponding to the clearance of the lever hook and the main contact is turned off. However, after this, an electric current flows further to the motor through the resistor connected to the auxiliary contact, so that the starter tries to run the engine and there is a fear of the auxiliary contact not being turned off. Thus, due to continuous current supply to the resistor, there is a danger of heat damage.
Further, in order to prevent the main contact from being turned on when the pinion abuts the end surface of the ring gear and to enable the auxiliary contact to be turned on without fail, it is necessary to achieve an improvement in terms of the dimensional accuracy of the components of the engine starter and the mounting precision for the pinion and the ring gear, resulting in an increase in cost.
Further, the arrangement of the resistor formed by bending a resistor wire with insulation coating into a meandering shape in the outer periphery of the switch coil leads to the following problem: when the engagement of the ring gear with the pinion is not effected successfully in a short period of time, a large electric current repeatedly flows through the resistor to cause the resistor to generate heat, thereby causing heat damage to the switch coil. Further, due to the heat generation of the switch coil, the resistance value of the resistor undergoes changes with passage of time, which may lead to a great variation in the RPM of the starter motor.
Further, the resistor is prepared by working a resistance wire, so that when changing the resistance value of the resistor and the rated current value, etc. in order to change the characteristics of the engine starter, it is necessary to change the diameter, etc. of the resistor wire. Thus, in some cases, the distance between the casing and the switch coil is smaller than the wire diameter, making it necessary to change the casing.